In hydropneumatic accumulator assemblies or in piston-cylinder assemblies, such as pneumatic working cylinders, in many cases knowing the exact position of the piston in the cylinder is desirable or necessary to be able to control a device. Moreover, knowing in hydropneumatic accumulator assemblies, how much gas is available to build up a back pressure in the accumulator element is important. Since the gas has a tendency to evaporate over time in the direction of the oil side, i.e. from time to time, the gas needs to be refilled, triggering a regular maintenance procedure.
Various solutions have been proposed in the past to determine the position of the piston. For example, installing an ultrasonic displacement measuring system on the oil side of a pressure accumulator is known. Such an ultrasonic single-channel system is sold by marco Systemanalyse and Entwicklung GmbH, Hans-Böckler-Str. 2, 85221 Dachau, Germany, under the name “ps/ulm/esd/a”. Sound signals are emitted by an ultrasonic transducer and reflected by a piston. The reflected sound wave is then received again by the ultrasonic transducer. In doing so, the acoustic signal propagates at a known propagation velocity in the oil, permitting the determination of the displacement of the piston from the signal propagation delay using the ultrasonic transducer. The disadvantage of this solution is the significant dependence of the sound propagation on the oil temperature and of gas bubbles undesirably occurring in the oil, for example, due to cavitation. Such gas bubbles affect the propagation of the sound signal, and thus, significantly distort the measurement result.
Furthermore, prior art includes arranging one or more ultrasonic transducers at the outside of a hydraulic piston-cylinder arrangement to be able to recognize in this way if a piston is in the immediate vicinity of the ultrasonic transducer. Such a device is sold by Sonotec Ultraschallsensorik Halle GmbH, Nauendorfer Str. 2, 06112 Halle (Saale), Germany, under the name “Sonocontrol 14”. Such devices are especially suitable for limit switches. A continuous position measurement of the piston is not possible in this case, even if several sensors are used at intervals.